Numerous lexemes that are reconstructable for Proto-Slavic have been identified as borrowings from the languages of various tribes that Proto-Slavic speakers interacted with in either prehistoric times or during their expansion when they first appeared in history in the sixth century (the Common Slavic period). Most of the loanwords come from Germanic languages, with other contributors being Iranian, Celtic, and Turkic. Slavic loanwords sparked numerous debates in the 20th century, some of which persist today.
Linguists Max Vasmer and Oleg Trubachyov compiled and published academic dictionaries on Slavic languages that are used worldwide in academia and are considered the most accurate sources for Slavic etymology. Another etymological dictionary written by G.P. Cyganenko takes a more modern look at the theories presented by Vasmer and others, and mostly explains origins for words that are most common and is not as extensive as the works of Vasmer or Trubachev.
Slavic and Iranian
Slavs in the Proto-Slavic era came into contact with various Iranian tribes, namely Scythians, Sarmatians, and Alans, who were present in vast regions of eastern and southeastern Europe in the first centuries CE. The names of two large rivers in the centre of Slavic expansion, Dnieper and Dniester, are of Iranian origin, and Iranian toponyms are found as far west as modern-day Romania.
For a long time there have been investigators who believe that the number of loanwords from Iranian languages in Proto-Slavic is substantial. However, other Slavists claimed that confirmed Iranianisms in Slavic are few in number, and Ranko Matasović has raised broad objections to the body of past Iranianist research. Antoine Meillet and André Vaillant explain the alleged lack of Iranianisms in Slavic: "the civilization of warrior and partially nomadic tribes, like Scythian and Sarmatian, could have exerted only a cursory influence on the patriarchal civilization of Slavs".
Matasović criticizes Zbigniew Gołąb's approach as "methodologically unacceptable", emphasizing that initial *x- in Slavic has several sources, some of which have been ascertained (like PIE *#ks-) and others which have not. Matasović recommends that instances of initial *x- in Slavic should first be explained by recourse to regular Slavic sound laws, and that Iranian should be proposed as a source if and only if the etymon has been attested in Iranian, and if and only if there is additional phonetic evidence to support the proposal.
Meillet and Vaillant considered that the semasiological development of the Proto-Slavic word for god was an Iranianism. In both Slavic and Indo-Iranian, the root that denotes deity also denotes wealth, share (Proto-Slavic *bagu > Common Slavic *bogъ) and Indo-Iranian (Old Persian baga, Sanskrit bhága).
One of the Iranian-Slavic lexical isoglosses is a lone adposition: Old Persian rādiy, OCS radi.
Slavic and Germanic
It is uncertain when Proto-Slavic speakers first came into contact with Germanic tribes: among Common Balto-Slavic words that have centum reflexes, none of them have typical Germanic sound-features. As for Baltic languages, all their prehistoric Germanic loanwords either come from Slavic or are borrowed from Old Norse or Proto-Norse; i.e., borrowed during a period well after Slavic prehistory (which ended 600 CE). The conclusion is that the speakers of Germanic must have lived far from the area of the subsequent spread of speakers of Proto-Balto-Slavic. Gothic loanwords into Slavic (as opposed to Baltic) occur much more frequently.